An aromatic polyimide is very excellent as a heat-resistant resin, and it is well known that the aromatic polyimide can be advantageously prepared in industry using a biphenyltetracarboxylic dianhydride and an aromatic diamine as starting materials, and possibly via production of a polyamic acid. The polyamic acid can be converted into a polyimide through imdation.
The biphenyltetracarboxylic dianhydride can be obtained by a process comprising the steps of dimerizing a phthalic acid diester such as dimethyl o-phthalate in the presence of a palladium catalyst, then subjecting the resulting dimer to various treatments such as purification, crystallization and hydrolysis to prepare a biphenyltetracarboxylic acid, and subsequently heating the biphenyltetracarboxylic acid for dehydration at a high temperature.
As for dehydration of the biphenyltetracarboxylic acid under heating at a high temperature, Japanese Patent Publication No. 57(1982)-15098 describes, for example, a method of heating the biphenyltetracarboxylic acid at a temperature of 100.degree. to 500.degree. C. in a nitrogen gas atmosphere under atmospheric pressure or reduced pressure such as a pressure of not lower than 40 mmHg to dehydrate the biphenyltetracarboxylic acid.
It has been found by the present inventors that the conventional method can hardly give a satisfactorily pure biphenyltetracarboxylic dianhydride. In more detail, when a biphenyltetracarboxylic dianhydride obtained by the conventional method is polymerized with an aromatic diamine to prepare an aromatic polyamic acid or polyimide, the viscosity of the aromatic polyamic acid or polyimide produced in the polymerization reaction liquid does not become sufficiently high. This means that an aromatic polyimide having a satisfactorily high molecular weight cannot be obtained.
The inventors have made study for discovering the reason why the viscosity of the polyamic acid or polyimide produced in the reaction liquid does not increase satisfactorily when the biphenyltetracarboxylic dianhydride obtained by the conventional method is used for the polymerization with an aromatic diamine. As a result, it has been found that increase of content of a biphenyltricarboxylic acid or its anhydride which is produced in the reaction and emigrates into the reaction product (i.e., biphenyltetracarboxylic dianhydride in the form of crystals) as impurity disturbs increase of the viscosity of the aromatic polyamic acid or polyimide produced in the polymerization reaction. Thus, it has been concluded that the biphenyltricarboxylic acid or its anhydride having emigrated into the crystals of a biphenyltetracarboxylic dianhydride disturbs production of an aromatic polyamic acid or polyimide having a satisfactorily high molecular weight.
Based on the above-mentioned finding, the present inventors have studied a process for preparing a high purity biphenyltetracarboxylic dianhydride which is almost free from contamination of a biphenyltricarboxylic acid or its anhydride. As a result of the study, the present inventors have discovered that crystals of a biphenyltetracarboxylic dianhydride containing almost no biphenyltricarboxylic acid or its anhydride, or containing an extremely small amount such as an amount of not more than 0.2 wt. %, can be obtained by heating crystals of a biphenyltetracarboxylic acid for dehydration under the specifically controlled heating conditions. The present inventors have also discovered that the employment of the above-obtained crystals of a biphenyltetracarboxylic dianhydride containing almost no biphenyltricarboxylic acid or its anhydride makes it possible to prepare an aromatic polyamic acid or polyimide having an increased high molecular weight.